1. Field of the Invention
The present invention relates to an information transmission method for transmitting location information such as traffic jams and accidents for example in a traffic information provision system and apparatus for exchanging location information using the method, and in particular to such a method and apparatus that can correctly report a location on a digital map.
2. Description of the Related Art
In recent years, the number of vehicles that have car-mounted navigation apparatus has been increasing rapidly. The car-mounted navigation system has a digital map database and is capable of displaying the map around the vehicle on a screen based on the latitude/longitude data received by a GPS receiver as well as displaying the travel locus and the result of search for a route to the destination on the map.
In Japan, digital map databases are prepared by several companies. The problem is that map data contains errors inherent in the reduced-scale map. The errors depend on individual digital maps. Some of the maps on the market drawn on the scale of 1:25000 contain errors of about 50 meters depending on the location.
Latitude/Longitude data obtained from a GPS receiver may contain errors of several tens of meters.
Some models of car-mounted navigation apparatus on the market receives traffic jam information and accident information from a traffic information provision system and displays the traffic jam and accident locations on the map or performs route search using such information as additional search conditions.
In a related art traffic information provision system, as shown in FIG. 7, traffic information is provided from a traffic information collecting center 71 that has local jurisdiction over an area to an information delivery center 72. Traffic information edited for each transmission medium including FM broadcasts, road beacons and cell phones is transmitted via respective media.
The traffic information collecting center 71 exchanges traffic information with a traffic information collecting center 78 in other areas to collect traffic information in a wide service area including the peripheral areas.
In the traffic information, for example, in case latitude/longitude data on the location is presented alone in order to report a traffic jam location or accident location, various errors are contained depending on the kind of digital map database of the car-mounted navigation apparatus as mentioned earlier. Thus, different accident locations on the road could be recognized by the car-mounted navigation apparatus retaining a digital map database from Company A and the car-mounted navigation apparatus retaining a digital map database from Company B.
In order to alleviate incorrect information transmission, as shown in FIG. 8A, crossings a and b on a road network are assumed as nodes, and a road c connecting the nodes as a link. Each node is assigned a node number uniquely representing the node (a=1111, b=3333). Each link is assigned a link number uniquely representing the link (c=11113333). In the digital map database of each company, the node numbers and link numbers thus assigned are stored, in correspondence with each crossing and road.
In traffic information provision service, a point on the road is represented by first specifying a link number then the distance in meters from the start of the link. For example, a display “Location 200 meters from the start of the road with link number=11113333” allows the user to obtain the location P on the same road by tracing the road with link number=11113333 as far as 200 meters from the node with node number 1111, irrespective of what digital map data is used by the car-mounted navigation system in question.
Node numbers and link numbers defined on a road network must be changed to new numbers in case a road d is constructed or a road is modified as shown in FIG. 8B. Such modifications to node numbers and link numbers require updating of digital map data from each company.
Construction of or modification to a road continue into the future. As long as on-road location representation is made using node numbers and link numbers, maintenance of digital map databases is permanently accompanied by huge workload and resulting costs.
The information delivery center must send information that complies with past node numbers and link numbers for a predetermined period, for example three to five years, as well as the information of the latest year, in order to support navigation systems sold in the past. This causes huge maintenance workload.